1. Field of the Invention
The present invention relates to wireless communication apparatus operating, for example, as an access point performing centralized control over a wireless communication area, more particularly to an apparatus with features that enable mobile stations to transmit fragment packets in a continuous series.
2. Description of the Related Art
Local area networks using wired transmission media have often employed the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) communication protocol to enable a plurality of terminal devices to transmit packets on a shared medium. CSMA/CD has been standardized by the Institute of Electrical and Electronics Engineers (IEEE) as IEEE standard 802.3 for media access in wired communication equipment. To transmit a packet, terminal equipment implementing the CSMA function senses the usage status of the medium and transmits the desired packet after confirming that the medium has been idle for a certain length of time.
Wired communication equipment implementing the CD function continues to sense the medium even after transmitting a packet. If the transmitted packet collides with a packet transmitted from another source, the wired communication equipment detects the collision and transmits the same packet again after a random time interval referred to as a back-off interval. The random back-off interval reduces the probability of another collision between retransmitted packets. The methods used for sensing the carrier and detecting collisions depend on technology which is only feasible in a wired communication environment, however, so a different protocol is necessary in a wireless communication environment.
One protocol suitable for a wireless communication environment is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). A version of CSMA/CA appears as a distributed coordination function (DCF) in IEEE standard 802.11, which concerns wireless local area networks. Wireless communication apparatus implementing the IEEE 802.11 distributed coordination function senses the medium before transmitting a packet, temporarily defers packet transmission if the medium is busy, waits for the medium to become idle, then waits for a further length of time equal to a fixed interval plus a random back-off interval, and transmits the packet if the medium remains idle during this further length of time. The fixed interval ensures a minimum spacing between transmitted packets; the random back-off interval reduces the probability of collisions between packets.
IEEE standard 802.11 also specifies a point coordination function (PCF) in which a single base station, referred to as an access point, controls communication among a set of mobile stations referred to as a basic service set (BSS). When the point coordination function is used, the access point has an independent right to transmit packets on the wireless communication medium to the stations in its BSS, but these stations cannot transmit packets without permission from the access point. Such permission is given a packet at a time in a process referred to as contention-free (CF) polling, by sending control information referred to as a CF-Poll.
When the access point has an outgoing packet of data to send to another station, it can transmit the packet and simultaneously poll the other station by setting CF-Poll information in the packet header. Alternatively, the access point can poll any station in the BSS by sending the station a packet including a CF-Poll with no packet body data. The access point maintains a list of CF-Pollable stations and polls these stations one at a time in their order on the list. The station that receives a CF-Poll from the access point can transmit the next packet after waiting for a comparatively short fixed interval, to which no random back-off interval is added.
The interval between packets transmitted under the point coordination function is accordingly shorter than the interval between packets transmitted under the distributed coordination function. This feature gives PCF transmission priority over DCF transmission. A CF-Poll, however, allows the polled station to transmit only one packet. A station cannot transmit a continuous series of packets unless it receives a continuous series of CF-Polls.
By having the access point control all packet transmission within a BSS, the point coordination function improves communication efficiency, as compared with the distributed coordination function, because all collisions between transmitted packets are avoided, and because the intervals between transmitted packets are shorter. Further information can be found in Japanese Unexamined Patent Application Publication No. 2002-300175, and in ‘Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications’, ANSI/IEEE Standard 802.11, 1999 Edition. Incidentally, the term ‘packet’ as used herein is synonymous with the term ‘frame’ used in the IEEE standard.
IEEE standard 802.11 also designates fragmentation and defragmentation functions. The fragmentation function divides a packet into a plurality of shorter fragment packets and transmits the fragment packets. The fragmentation function makes the retransmission of packets in which a bit error occurs during transmission more efficient, because only one fragment of the packet has to be retransmitted. The defragmentation function reassembles the fragment packets to reconstruct the original packet. Since it may not be possible to receive all of the fragment packets constituting the original packet, a defragmentation timeout limit (MaxReceiveLifetime) is set as a parameter of the defragmentation function. A timer at the receiving station is started at the reception of the first fragment packet. If the reconstruction of the original packet is not completed within the timeout limit, the receiving station discards the packet.
When the fragmentation and defragmentation functions are used with distributed coordination, a mobile station does not need to receive permission from the access point to transmit packets, so it can transmit a plurality of fragment packets in a continuous series. More accurately, the fragment packets and the answering acknowledgment packets from the receiving station alternate with one another in a continuous series. This capability is guaranteed by a provision in IEEE standard 802.11 that lets a station omit the random back-off interval after transmitting a fragment packet. Normally this allows all fragment packets to be received within the defragmentation timeout limit.
When the fragmentation and defragmentation functions are used with point coordination, however, a problem occurs. Because a mobile station receives permission to transmit only one fragment packet at a time, and because permission is granted according to a polling list, there is no guarantee that a station with fragment packets to transmit will be able to transmit the fragment packets in a continuous series. Normally, the station will not be able to do so, because the access point must poll all of the stations on its polling list before it can send a consecutive series of CF-Polls to any one station. A station that receives a fragment packet transmitted under PCF control may therefore have to wait for a considerable time before receiving the next fragment packet, making defragmentation timeout a significant problem during PCF periods.